The present invention relates to an electronic switch.
The use of electronic switches involves the difficulty that, because of the physical properties of the semiconductor devices used, e.g., MOS transistors, inductively or capacitively caused changes in the potential on an input may result in an abnormal switching behaviour of the switch. This is particularly undesirable if a plurality of switches are realized using integrated circuit technology, because other components will be disturbed, too, and if the switches are used as switching elements in switching networks, crosstalk and disturbing noise will occur.
In an N-MOS transistor, during normal operation, the source-substrate and drain-substrate junctions must be reverse-biased, and drain and source must be more positive than the substrate. The MOS transistor is controlled by opening and closing the channel by means of the gate-to-source voltage. The channel is open, i.e., the switch is closed, when the gate-to-source voltage exceeds the threshold voltage of the MOS transistor.
If, however, the potential of the input becomes negative, the source-substrate np junction will open, and substrate current will flow; with the drain-source junction reverse-biased, an npn transistor can form, with emitter = source, base = substrate, and collector = drain. As a result, the channel is indirectly rendered conductive, i.e., the switch is inadvertently turned on. The channel is also rendered conductive when the negative potential of the source exceeds the threshold voltage.
The object of the invention is to avoid these disturbances of the switch due to a negative potential at the input.